SYDNEY, Nov 8 (Reuters) - The world's biggest fibre cement maker, James Hardie Industries PLC, cut its earnings guidance on Thursday and said the future of a U.S. home building boom propelling its profit was uncertain, sending its shares tumbling to a more than two-year low.

Buoyant building, especially of houses, in the United States has for years underpinned growth at James Hardie and similarly exposed peers such as building products firm Boral Ltd and hardware seller Lowe's Companies Inc.

But with official figures and James Hardie's own sales pointing to a slowdown, the firm appeared to call time on the boom, dropping its full-year guidance window by $20 million and saying even that depended on a housing bounce-back it rates as uncertain.

James Hardie shares had their worst day on Thursday since the 1987 stock market crash, plunging as much as 15 percent to their lowest in 30 months and wiping out A$1.3 billion ($946 million) in market value - and auguring badly for its rivals.

"The big hope that has kept it motoring along over the last five years has been the U.S.," said Evan Lucas, chief market strategist at fund manager InvestSmart.

"They're alluding to the fact that they're expecting a little bit of rocky trading," he said. "Everyone was expecting the U.S. to continue to be the white knight that it is and that was disappointing."

The U.S. housing market, once an engine room for growth, has this year underperformed a robust economy. September housing starts dropped further than expected, with economists blaming the slowdown - exacerbated by a hurricane hitting the Carolinas - on low inventories and rising mortgage rates.

James Hardie said its pace of cement volume growth in the U.S., which comprises 91 percent of total sales, held at 5 percent, while the market had expected growth over 6 percent.

Profit nevertheless rose 17 percent to $160.8 million for the six months to Sept. 30, boosted by the acquisition of a fibreboard panel maker.

However its profit margin was squeezed as freight and raw material costs rose, and Chief Executive Louis Gries said he expected that would persist through the year, prompting the guidance cut.

The company lowered its full-year operating profit forecast to a range of $280 million to $320 million, from $300 million to $340 million made in August.

That was below analyst expectations for annual profit of $313 million to $335 million, and the company said the revised target still depends on housing starts growing.

"Although U.S. housing activity has been improving, market conditions remain somewhat uncertain and some input costs remain volatile," James Hardie said in a statement.

The Ireland-headquartered firm reported net sales of $1.30 billion for the half year, from $1.03 billion a year prior, and declared a half-yearly dividend of 10 cents per share. ($1 = 1.3740 Australian dollars)